Thomas Paine’s "Common Sense" is credited with having precipitated the move for independence. In fact, the exact nature of the American cause would have been rather hard to define in 1775 or early 1776. Clearly the Americans wanted the English to stop abusing them, as they saw it, but how was fighting a war supposed to achieve that end? What would constitute victory? As long as they were still British subjects, they would still be subject to British law, and by 1775 it was unlikely that Parliament would grant them any real form of self government. As the Declaratory Act of 1766 had made clear, Parliament claimed the right to govern the colonies "in all cases whatsoever." Since achieving quasi-independence was an unrealistic hope, therefore, the only thing that finally did make sense was American independence, a case made very powerfully by Thomas Paine.
Here are excerpts from Common Sense.

Society in every state is a blessing, but Government, even in its best state, is but a necessary evil; in its worst state an intolerable one: for when we suffer, or are exposed to the same miseries by a Government , which we might expect in a country without Government, our calamity is heightened by reflecting that we furnish the means by which we suffer. Government, like dress, is the badge of lost innocence; the palaces of kings are built upon the ruins of the bowers of paradise. For were the impulses of conscience clear, uniform and irresistibly obeyed, man would need no other lawgiver; but that not being the case, he finds it necessary to surrender up a part of his property to furnish means for the protection of the rest; and this he is induced to do by the same prudence which in every other case advises him, out of two evils, to choose the least. Wherefore, security being the true design and end of government, it unanswerably follows that whatever form thereof appears most likely to ensure it to us, with the least expense and greatest benefit, is preferable to all others.
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I draw my idea of the form of government from a principle in nature which no art can overturn, viz. that the more simple any thing is, the less liable it is to be disordered, and the easier repaired when disordered; and with this maxim in view I offer a few remarks on the so much boasted constitution of England. That it was noble for the dark and slavish times in which it was erected, is granted. When the world was overrun with tyranny the least remove therefrom was a glorious rescue. But that it is imperfect, subject to convulsions, and incapable of producing what it seems to promise, is easily demonstrated. …
There is something exceedingly ridiculous in the composition of Monarchy; it first excludes a man from the means of information, yet empowers him to act in cases where the highest judgment is required. The state of a king shuts him from the World, yet the business of a king requires him to know it thoroughly; wherefore the different parts, by unnaturally opposing and destroying each other, prove the whole character to be absurd and useless

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I have heard it asserted by some, that as America has flourished under her former connection with Great-Britain, the same connection is necessary towards her future happiness, and will always have the same effect. Nothing can be more fallacious than this kind of argument. We may as well assert that because a child has thrived upon milk, that it is never to have meat, or that the first twenty years of our lives is to become a precedent for the next twenty. But even this is admitting more than is true; for I answer roundly, that America would have flourished as much, and probably much more, had no European power taken any notice of her. The commerce by which she hath enriched herself are the necessaries of life, and will always have a market while eating is the custom of Europe.
But she has protected us, say some. That she hath engrossed us is true, and defended the Continent at our expence as well as her own, is admitted; and she would have defended Turkey from the same motive, viz. for the sake of trade and dominion.

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But Britain is the parent country, say some. Then the more shame upon her conduct. Even brutes do not devour their young, nor savages make war upon their families.

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Common sense excerpts